switch to function component with-react-intl example  (#14217)
switch to function component with-react-intl example  (#14217)

The example was using class-based component for custom _app, switched to functional component approach.


**Sidenote:** The existing code didn't gave an error when navigated to a new page using the navbar
![Screenshot 2020-06-16 at 2 47 52 PM](https://user-images.githubusercontent.com/11258286/84760988-6cbebd80-afe6-11ea-9b7f-98aca7404895.png)

JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,flybayer/next.js,flybayer/next.js,azukaru/next.js,zeit/next.js,zeit/next.js,azukaru/next.js,zeit/next.js,JeromeFitz/next.js